Alkynes terminal, addition

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

Terminal alkynes readily react with coordinatively unsaturated transition metal complexes to yield vinylidene complexes. If the vinylidene complex is sufficiently electrophilic, nucleophiles such as amides, alcohols or water can add to the a-carbon atom to yield heteroatom-substituted carbene complexes (Figure 2.10) [129 -135]. If the nucleophile is bound to the alkyne, intramolecular addition to the intermediate vinylidene will lead to the formation of heterocyclic carbene complexes [136-141]. Vinylidene complexes can further undergo [2 -i- 2] cycloadditions with imines, forming azetidin-2-ylidene complexes [142,143]. Cycloaddition to azines leads to the formation of pyrazolidin-3-ylidene complexes [143] (Table 2.7). 

As a further development of the hydrotelluration of alkynes the addition of diisobutyl-aluminium tellurolate to terminal acetylene was investigated. 

After the discovery of the first terminal vinylidene-metal complex in 1972, it was established that the stoichiometric activation of terminal alkynes by a variety of suitable metal complexes led to 1,2-hydrogen transfer and the formation of metal-vinylidene species, which is now a classical organometallic reaction. A metal-vinylidene intermediate was proposed for the first time in 1986 to explain a catalytic anti-Markovnikov addition to terminal alkynes. Since then, possible metal-vinylidene intermediate formation has been researched to achieve catalytic regiose-lective formation of carbon-heteroatom and carbon-carbon bonds involving the alkyne terminal carbon. 

Sonogashira protocol.The alkynylzinc reagent can also be prepared in situ form terminal alkynes by addition of ZnCl2 as a co-catalyst. ... 

There are numerous examples of the addition of HBr to alkynes.10 Addition to acetylene is difficult, tends to produce mixtures of bromide products, and requires a catalyst.94,10 1 22 Simple terminal alkynes react with HBr in the absence of peroxides to produce mixtures of 2-bromo-l -alkenes and geminal dibromides (equation 110).85,159 The latter can be produced in high yield when excess HBr is used. 60... 

Addition of HCN to —C C— nitriles. In the presence of this nickel(O) catalyst, HCN adds stereospecifically to both terminal and disubstituted alkynes. The addition is also regioselective. 

Unlike previous alkyne-aldehyde additions [23], the generation of an alkynyl carbanion is unlikely owing to the large pK, difference between the terminal acetylene and the solvent water [24]. A mechanism was proposed involving the simultaneous activation of the C-H bond of alkyne by the ruthenium catalyst and the aldehyde carbonyl by the indium ion. The ruthenium intermediate then underwent Grignard-type addition followed by an in situ hydrolysis in water to give the desired carbonyl addition product and regenerated the ruthenium and indium catalysts to catalyze further reactions (Fig. 3). 

The addition of diboron compounds to alkynes is an excellent method for the synthesis of c -diboryl alkenes (Scheme 2-11) [34], The reaction is catalyzed by Pt(PPh3)4 at 80 and works well not only with terminal but also with internal alkynes. The addition of the Si-B [35] or Sn-B [36] bonds to alkynes gives mixed-metal alkenylboron reagents which have potential ability for use in the stepwise double cross-coupling reaction at both metallated carbons. 

Although exclusive terminal addition of hydrosilanes to 1-alkynes is claimed , careful investigation discloses that substantial amounts of side product(s) are formed. Typically, the hydrosilylation of hex-l-yne with trichlorosilane (1 1) in the presence of H2PtCl6 gives a mixture of 1- and 2-trichlorosilylhex-l-ene (78 22) ... 

Carbohalogenation of various terminal or internal alkynes, via addition of perfluoroalkyl iodides or bomides, is catalyzed by carbonyl complexes of iron, cobalt or ruthenium. In this case, dichlorotris(triphenylphosphane)ruthenium(II) is not active as a catalyst. rram-Addition products are usually obtained in good yield under mild reaction conditions36. 

Linstrumelle and co-workers reported that vinyl and aryl halides or triflates react very rapidly with terminal alkynes, without addition of copper salt, and lead to high yields of eneynes and aryl acetylenes by using Pd(PPh3)4 as a catalyst. The nature of the amine is critical for the success of the coupling (Scheme 3, [Pd] I). However,... 

Nurmi et have attached a functional thiol by Michael addition TEC at a polymer chain end, followed by CuAAC reaction of the pendant alkynes from the monomer units, further demonstrating orthogonality between CuAAC and TEC. Yu etal have synthesized alkene- and alkyne-terminated PNIPAAm for reaction by thiol-ene, or the closely related thiol-yne, chemistry. These polymers were then functionalized with commercially available thiols, and NMR spectroscopy showed complete consumption of the allyl or propargyl protons with concomitant appearance of unique resonances for the added thiols. Integration of these peaks demonstrated that thiol-yne chemistry provided full double functionalization, while thiol-ene provided full stoichiometric conversion. 

Several hundred examples of vinylidene complexes have been prepared. Vinylidene complexes have been prepared by rearrangement of alkyne complexes, additions of acid or base to acetylide complexes, by deprotonation of carbyne complexes, by dehydration of acyl complexes, and by ot-hydrogen shifts from vinyl complexes. Syntheses from alkjme and from acetylide complexes are most common. A complex of a terminal alkyne and a transition metal can exist as an alkyne complex or as a vinylidene complex. Although the free vinylidene is much higher in energy than the free alkyne, the vinylidene complex is often more stable tlnan the alkyne complex. Vinylidene complexes are most often obtained with late transition metals because this tautomer possesses less repulsion between the filled (i-orbitals of the metal and the filled ir-orbitals of the ligand. 

We have now seen how to prepare both terminal and internal alkynes from acetylene and substituted acetylenes, and we have seen several common reactions of alkynes, including addition (HX, X, and H2O), hydroboration-oxidation, and reduction. Now let us move a step farther to consider what might be called the art of organic synthesis. 

Alkynes undergo addition reactions in much the same way that alkenes do, although their reactivity is typically less than that of alkenes. In addition, terminal alkynes (RC=CH) are weakly acidic and can be converted into their corresponding acetylide anions on treatment with a sufficiently strong base. 

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